NATURAL CLASSIFICATION OF THE TRILODITES 125 



cephalon in front of the genal angles. Genera having i\m 

 structure are here placed in the order Peopakia. 



Several genera, as Calyvimene and Triarthrus, have been 

 described as having the facial sutures beginning at or cutting 

 the apex of the genal angle, thus making it indeterminate 

 whether they should be classed with the Opisthoparia or 

 Proparia. It will be found, however, that some species of 

 these genera leave no doubt as to the anterior or posterior 

 position of the suture. The small genal spines of Oalymmene 

 callieephala Green are situated on the ends of the fixed- 

 cheeks, while similar but larger spines in Triarthrus spinosus 

 Billings are on the free-cheeks, making the former belong to 

 the Proparia and the latter to the Opisthoparia. 



Application of Principles for Arrani/ement of Families 

 and Genera. 



The remaining characters to be noticed have chiefly family 

 and generic values, and naturally follow the preceding dis- 

 cussions. They are of great assistance both in determining 

 the place of a family in an order, and the rank and genetic 

 position of a genus in a family. 



There is very satisfactory evidence that the eyes have 

 migrated from the ventral side, first forward to the margin 

 and then backward over the cephalon to their adult position. 

 The most primitive larvre should therefore present no evi- 

 dence of eyes on the dorsal shield. Just such conditions are 

 fulfilled in the youngest larva of Ptychoparia, Solenopleura, 

 and Liostracns. The eye-line is present in the later larval 

 and adolescent stages of these genera, and persists to the 

 adult condition. In Sao it has been pushed forward to the 

 earliest protaspis, and is also found in the two known larval 

 stages of Triarthrus. Sao retains the eye-line throughout 

 life, but in Triarthrus the adult has no trace of it. A 

 study of the genera of trilobites shows that this is a very 

 archaic feature, chiefly characteristic of Cambrian genera, 

 and only appearing in the primitive genera of higher and 



